With regard to the running controls of automobiles, electronic control of various components is widely used. Regarding the intake air quantity to be supplied to an internal combustion engine, an electric air flow control device controlling by using optimized electric signals after each driving condition is converted into electric signals tends to be standardized, instead of a mechanical air flow control device which controls mechanically by being linked to a connecting structure such as a wire for transmitting the pushing amount on the accelerator by a driver.
In recent years, the emission control as an environmental measure has become stricter and more complicated, and electric control tends to be often used even for diesel type internal combustion engines, which controlled little the exact intake air flow conventionally.
The electric air flow control device has a motor or the like for controlling the rotation of a combined gear mechanism with a required gear ratio, connected with a shaft end of an air flow control valve (throttle valve) receiving reaction force of a spring. As the feature of the air flow control valve, a butterfly valve structure for open and close control in a round-shaped passage is common and it carries out repetitive open and close control with a maximum rotary motion angle of approximately 90 degrees.
Even when the throttle valve for controlling an air flow receives the open and close control from the position corresponding to the fully closed passage to the fully open passage state, the number of revolutions of the motor for driving is usually about four, because the angle of rotary motion of the throttle valve is about 90 degrees, and thus the motor has no opportunity of continuous high speed rotation like common motors.
In view of a continuous constant driving state on the expressways or the like, the driving motor only receives control for a slight forward-reverse rotation close to the standstill state, though a slight open and close control of the throttle valve is conducted in response to the accelerator operation. (Although the motor is required to issue a balancing force to maintain the throttle valve opening to secure the targeted driving state, it has almost no chance of a continuous rotating movement compared with motors for the general industrial equipment.)
When the load environment of the motor that is mounted on an internal combustion engine and is a drive source of the electric air flow control device is considered, it is understood that an environment of vibration transmitted from the internal combustion engine, from the road surface or the like exists as well as rotating speed of the motor itself and a low and high temperature environment in view of the state from being left in a cold district to the continuous driving. In the vicinity of both ends of the motor rotor shaft, a rolling bearing represented by a ball bearing or a needle bearing, or a cylindrical slide bearing structure is provided for maintaining a stable movement of the motor rotor.
When the operating conditions of vehicle motors are divided into two groups, one has a specification of a continuous high-speed operation for driving components such as cooling water circulation pumps, cooling fans and air conditioner fans and the other has an operation specification for controlling each related equipment (for example, for an electric brake control and an electric air flow control device) having almost no continuous rotation.
In the latter case, the positional relationship between the motor rotating shaft and the bearing changes in only an extremely limited narrow range and in the case of use of a rolling bearing such as a ball or needle bearing, the rotation of balls or needs themselves is not expected, and the rolling members and the track surfaces of the inner and outer rings have point contact or line contact at almost the fixed positions. When a high vibrational load environment specific to the internal combustion engine (vehicle) is applied, a high stress (surface pressure) occurs at the microscopic contacting portion between the rolling member and the inner and outer ring track surfaces, resulting in malfunctioning damage on the rolling members or the tracks.
For motors with a long service life designed in view of the peculiarity of the motor operation state having almost no continuation of the high speed rotation and the severe vehicle vibrational environment, cylindrical slide bearings of the copper alloy system are often employed in consideration of the materials of the rotating shaft (generally iron-based materials). In particular, productivity and lubricating ability of the bearing members are considered and most of them have designed structures in which copper-alloy-based sintered metals are impregnated with lubricating oil. High reliability for supporting the rotation stably in a long term is demanded from such sintered of retaining bearings and various proposals regarding the metal matrix of copper alloy as a base material and lubricating oil for impregnation are made (NPLS 1 to 3 and PTL 1).
NPL 1. proposes each kind of a Cu—Sn system, Cu—Sn—C system, Cu—Sn—Ni system, Cu—Sn—C—P system and Cu—Sn—MoS2 system as materials for sintered oil retaining bearings. NFL 2 highlights a Cu—Sn—MoS2-based material HZ18 for a bearing for an electronic control throttle and by adding solid lubricating performance by addition of molybdenum disulfide and by adopting a fluorine system as a lubricating oil for impregnation and a PTFE as a thickener for the lubricating oil, a bearing with a high quietness and wear resistance can be provided even at a high temperature environment of 150° C. A comparison of the wear amount property is disclosed as a comparison with conventional materials in an abrasion test at a rotating speed of 3000 r/min, with a radial load of 9.8 N, and at an ambient temperature of 160° C. as the evaluation conditions to show that the wear amount after approximately 450 hours reduces to ⅓ of that of the conventional material.
NPL 3 proposes a Cu—Sn system, Cu—Sn—C system, Cu—Sn—Ni—C system and others as copper-alloy-based bearing materials, and PTL 1 introduces, as a specific proposal for the purpose of enhancement of the mechanical property of the copper-alloy-based sintered base material and the lubricating performance of impregnating oil, a material configured such that the metal matrix contains 2-6% by mass of Ni, 7-13% by mass of Sn, and Cu and foreign matters as the remainder, and perfluoropolyether as lubricating oil is used for impregnation in holes of a porous sintered body in which a Ni—Sn—Cu alloy phase of 20-100 μm is dispersed in a Cu—Sn alloy base, in order to achieve an excellent slide property.